Many semiconductor light-emitting devices produce light through radiative recombination of electrons and holes at a p-n junction. Both bulk materials and quantum well structures may be used to form p-n junctions. Quantum well p-n heterojunctions can produce high light-emitting efficiency in this type of light-emitting devices. In operation, a driving electrical current is applied to the p-n junction to inject carriers (i.e., electrons and holes) with energy above their respective equilibrium level into the p-n junction. A large portion of the electrons and holes recombine to release the excessive energy as light. The remaining electrons and holes recombine through nonradiative processes to produce heat. From another point of view, a majority of the electrons that are excited from the valence band to the conduction band by absorbing the energy from the driving current radiate photons by decaying back to the valence band.
Examples of such light-emitting devices include light-emitting diodes ("LEDs") and diode lasers. LEDs operate based on the spontaneous emission of photons, and diode lasers operate based on the stimulated emission of photons and population inversion. The structure of a diode laser is usually more complex than that of a LED since an optical cavity is required in a diode laser to provide necessary optical feedback for laser oscillations.
It is desirable to generate as much light as possible for a given amount of driving current in both LEDs and diode lasers. This aspect of a LED or diode laser can be characterized by electrical-to-light conversion efficiency, which is defined as the ratio of the output light power to the injected electrical power. In practical devices, increasing the electrical-to-light conversion efficiency can also reduce the heat caused by the remaining electrical energy from the injected driving current that is not converted into light. Low thermal dissipation is particularly desirable in manufacturing compactly integrated photonic circuits.
The electrical-to-light conversion efficiency of a LED or diode laser has an upper limit defined by the internal quantum efficiency of a p-n junction, which is the rate of emission of photons divided by the rate of supply of electrons (or holes). Choice of semiconductor materials, dopants and respective doping concentrations may be used to increase the quantum efficiency. Use of quantum well structures rather than bulk materials to form a p-n heterojunction, for example, is one approach to improve the quantum efficiency.
The device structure of a LED or diode laser may also affect the electrical-to-light efficiency. For a given p-n junction, the electrical-to-light efficiency is mainly determined by the device structure since the quantum efficiency is essentially fixed. Various structures for LEDs and diode lasers have been developed to improve the electrical-to-light efficiency. One effort in this area is to confine the driving current to a small spatial region at or near the active p-n junction in order to increase the current density in the p-n junction. This results in an increase in the rate of supply of electrons (or holes) to the p-n junction, and thereby increases the rate of photon emission.
In diode lasers, the electrical-to-light efficiency can be represented by a laser threshold current at which a population inversion is created between the conduction and valence bands. At this threshold current, the optical gain caused by the driving current is equal to the total optical loss. A low threshold current indicates a high electrical-to-light conversion efficiency. For a given active p-n junction with a fixed thickness and material compositions, the laser threshold current density is also given. Thus, confining current spatial distribution to a smaller region near or at the p-n junction increases the corresponding current density and effectively reduces laser threshold current.